1. Field of the Invention
This invention relates to a light emitting device and a lighting apparatus using the light emitting device.
2. Description of the Related Art
Incandescent bulbs have long been used as a light source for lighting, and in more earlier times before the incandescent bulbs are invented, natural combustion light such as candles or torches had been used. Incandescent bulbs which illuminate based on resistance heat generation of the filaments suffer from a relatively short service life and a low efficiency due to a large Joule heat loss. They are also disadvantageous in that a large amount of infrared radiation emitted together with visible light during the light emission, and this considerably raises temperature of the light source. Natural combustion lights suffer from further shorter service life, and require labor-consuming maintenance works due to production of large amount of combustion products such as soot and carbon dioxide.
On the other hand, fluorescent lamps have been used as a general light source for lighting other than incandescent bulbs. The fluorescent lamp generates hot electrons in an evacuated glass tube filled with a trace amount of mercury gas by applying a high voltage between the electrodes enclosed therein, allows mercury to be excited by the hot electrons so as to emit ultraviolet radiation, and allows a fluorescent material coated on the inner surface of the glass tube to illuminate based on photoluminescence using the ultraviolet radiation. While the emission having a variety of spectra can be obtained by selection of fluorescent materials to be used, most popular fluorescent material is calcium halophosphate (3Ca3(PO4)2·CaFCl/Sb, Mn), where white light having various color temperatures can be obtained by typically adjusting ratios of F and Cl, or that of Sb and Mn. The fluorescent lamps have replaced the incandescent bulbs in most part of the applications where use of the incandescent bulbs was prevailing in the past.
The fluorescent lamps, however, still suffer from the following drawbacks:                the service life tends to terminate relatively within a short time due to evaporative consumption of the electrodes since ultraviolet radiation is generated by cathode discharge;        although power consumption is smaller than that of incandescent bulbs, it is still not ignorable because high voltage is necessary;        it is expected to be disused from the viewpoint of environmental preservation because disposal thereof releases mercury filled in the glass tube as a ultraviolet emission source;        leakage of ultraviolet radiation tends to cause yellowing of papers or the like; and        phosphorescent light obtained from calcium halophosphates is substantially white, but the spectrum thereof largely differs from that of natural light (sunlight), and thus raises a problem of color rendering properties. Even an effort of realizing more excellent color rendering properties by combining narrow-range light emissions in three wavelength regions for red, green and blue (RGB) still remains a large gap to the natural light in terms of the color rendering properties due to sharp peaky bright line spectra of the fluorescent materials of the individual colors.        
Light emitting diodes are also known as a light emitting source. Light emitting diodes, however, have only limited emission wavelength regions which are governed by band gap values of semiconductors to be used, and this makes it impossible to obtain an emission spectrum covering a wide visible light band, and emission from most of them is only almost monochromatic. There have thus been almost no applications of the light emitting diodes in the field of general lighting where color rendering properties are required. Recent development of a high-luminance blue emission device using InGaAlN-base compounds has made it possible to develop various light emitting devices capable of emitting a variety of mixed colors, which are based on combination of light emitting devices causing red, green and blue monochromatic lights corresponding to three principal colors of light, intended for use in lighting. This kind of devices, however, raise a problem that they can produce only extremely discrete emission spectra characterized by sharp peaky bright lines, and consequently that they are poor in color rendering properties for the intermediate wavelength regions.
It is therefore a subject of this invention to provide a light emitting device capable of readily produce a pseudo-continuous spectrum covering a wide wavelength region at low costs, and of totally solving various problems which have resided in the conventional light sources, and a lighting apparatus using this device.